Backtracking

let dir = promptDirection("Where?"); console.log("You chose ", dir); break; } catch (e) { if (e instanceof InputError) { console.log("Not a valid direction. Try again."); } else { throw e;

This will catch only instances of InputError and let unrelated exceptions through. If you reintroduce the typo, the undefined binding error will be properly reported.

Assertions

Assertions are checks inside a program that verify that something is the way it is supposed to be. They are used not to handle situations that can come up in normal operation but to find programmer mistakes.

If, for example, firstElement is described as a function that should never be called on empty arrays, we might write it like this:

function firstElement(array) { if (array.length == 0) { throw new Error("firstElement called with []");

} return array[0];

Now, instead of silently returning undefined (which you get when reading an array property that does not exist), this will loudly blow up your program as soon as you misuse it. This makes it less likely for such mistakes to go unnoticed and easier to find their cause when they occur.

I do not recommend trying to write assertions for every possible kind of bad input. That’d be a lot of work and would lead to very noisy code. You’ll want to reserve them for mistakes that are easy to make (or that you find yourself making).

145

Summary

Mistakes and bad input are facts of life. An important part of programming is finding, diagnosing, and fixing bugs. Problems can become easier to notice if you have an automated test suite or add assertions to your programs.

Problems caused by factors outside the program’s control should usually be handled gracefully. Sometimes, when the problem can be handled locally, special return values are a good way to track them. Otherwise, exceptions may be preferable.

Throwing an exception causes the call stack to be unwound until the next enclosing try/catch block or until the bottom of the stack. The exception value will be given to the catch block that catches it, which should verify that it is actually the expected kind of exception and then do something with it. To help address the unpredictable control flow caused by exceptions, finally blocks can be used to ensure that a piece of code always runs when a block finishes.

Exercises

Retry

Say you have a function primitiveMultiply that in 20 percent of cases multiplies two numbers and in the other 80 percent of cases raises an exception of type MultiplicatorUnitFailure. Write a function that wraps this clunky function and just keeps trying until a call succeeds, after which it returns the result.

Make sure you handle only the exceptions you are trying to handle.

The locked box

Consider the following (rather contrived) object:

const box = { locked: true, unlock() { this.locked = false; }, lock() { this.locked = true; }, _content: [], get content() { if (this.locked) throw new Error("Locked!"); return this._content;

146